感谢：
http://halisway.blogspot.com/2007/06/reclaiming-lob-space-in-oracle.html
http://stackoverflow.com/questions/18367444/how-to-drop-oracle-lob
LOB字段是单独存储的，而且用delete是无法回收lob占用的空间。
SQL> create table lob_test (id number, data blob);
Table created.
-- 向lob_test中导入数据
SQL> @lobload
PL/SQL procedure successfully completed.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 131072
SYS_LOB0000316067C00002$$ LOBSEGMENT 134217728
-- 现在可以看到LOB是单独存储的
SQL> delete from lob_test;
500 rows deleted.
SQL> commit;
Commit complete.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 327680
SYS_LOB0000316067C00002$$ LOBSEGMENT 134217728
-- delete后，lob占用的空间没有释放
-- 再次导入
SQL> @lobload
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 393216
SYS_LOB0000316067C00002$$ LOBSEGMENT 260046848
-- lob对应的segment 继续增长
-- 再次删除
SQL> delete from lob_test;
500 rows deleted.
SQL> commit;
Commit complete.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 655360
SYS_LOB0000316067C00002$$ LOBSEGMENT 260046848
SQL> alter table lob_test modify lob(data) (shrink space);
Table altered.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 655360
SYS_LOB0000316067C00002$$ LOBSEGMENT 65536
SQL> alter table lob_test modify lob(data) (shrink space);
Table altered.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 655360
SYS_LOB0000316067C00002$$ LOBSEGMENT 65536
--使用alter后 lob占用的空间被回收。
注意一
：在drop表的时候，如果不加purge ，lob segment一样不会被删除
删除lob_test前：
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 655360
SYS_LOB0000316067C00002$$ LOBSEGMENT 134217728
SQL> drop table lob_test;
Table dropped.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
BIN$+oFtu0VUcIzgQH+HqwZhEw==$0 TABLE 131072
SYS_IL0000316067C00002$$ LOBINDEX 655360
SYS_LOB0000316067C00002$$ LOBSEGMENT 134217728
依然可以看到lob字段，证明不加purge是不法释放lob segment的
在回收站中，清理lob_test表
SQL> purge table lob_test;
Table purged.
SQL> select segment_name, segment_type, bytes from user_segments;
no rows selected
证明使用purge可以释放lob segment。
注意二：
一开始的建表语句是：
create table lob_test (id number, data blob);
如果，建表时指定lob的segment，在drop不加purge时，lob的segment可以直接被删除，放入回收站中。
create table lob_test22(id number, data blob)
lob (data) store as data_blob_segment;
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
LOB_TEST22 TABLE 131072
SYS_IL0000316073C00002$$ LOBINDEX 131072
DATA_BLOB_SEGMENT LOBSEGMENT 134217728
SQL> drop table lob_test22;
Table dropped.
SQL> select segment_name, segment_type, bytes from user_segments;
SEGMENT_NAME SEGMENT_TYPE BYTES
------------------------------ ------------------ ----------
BIN$+oFtu0VWcIzgQH+HqwZhEw==$0 TABLE 131072
SYS_IL0000316073C00002$$ LOBINDEX 131072
BIN$+oFtu0VVcIzgQH+HqwZhEw==$0 LOBSEGMENT 134217728
SQL> select object_name, ORIGINAL_NAME from user_recyclebin;
OBJECT_NAME ORIGINAL_NAME
------------------------------ --------------------------------
SYS_IL0000316073C00002$$ SYS_IL0000316073C00002$$
BIN$+oFtu0VVcIzgQH+HqwZhEw==$0 DATA_BLOB_SEGMENT
BIN$+oFtu0VWcIzgQH+HqwZhEw==$0 LOB_TEST22
附：
-bash-3.2$ cat lobload.sql
DECLARE
src_file BFILE := bfilename('MY_DIR', 'data.dat');
dst_file BLOB;
lgh_file BINARY_INTEGER;
cur_id NUMBER(10);
BEGIN
FOR i IN 1..500
LOOP
if (mod (i,10) = 0) then
dbms_output.put_line(i);
end if;
INSERT INTO lob_test22(id,data) VALUES(lob_test_seq.nextval,empty_blob())
RETURNING id into cur_id;
commit;
-- lock record
SELECT data INTO dst_file FROM lob_test22 WHERE id=cur_id FOR UPDATE;
dbms_lob.fileopen(src_file, dbms_lob.file_readonly);
lgh_file := dbms_lob.getlength(src_file);
dbms_lob.loadfromfile(dst_file, src_file, lgh_file);
dbms_lob.fileclose(src_file);
END LOOP;
END;
/
--这里的MY_DIR 是指数据库中的directory。如果使用create directory my_dir as '/tmp'; 那么这里就要求必须使用大写的MY_DIR。
移动LOB字段的哦其他表空间：
ALTER TABLE CENTER_ADMIN.NWS_NEWS
MOVE LOB(ABSTRACT)
STORE AS (TABLESPACE lob_test);